1. Field of the Invention
The present invention relates generally to wireless digital communication systems and particularly to receivers employed in such systems and including iterative decoder.
2. Description of the Prior Art
In wireless digital communication systems, the orientation of the receive antenna can have a major impact on receiver performance. Some antenna orientations may render the signal unusable by the receiver. One well-known approach to overcome this problem is the use of “antenna diversity.” This involves receiving the same transmit signal with two or more receive antennas, and combining the signals in the receiver. The combined signal in this type of receiver generally has a higher probability of being decoded correctly than either of the constituent signals alone.
The optimum approach for combining signals in a diversity receiver is called “maximal ratio combining.” This approach is well-known in the art, and dates back to the days of analog communications and vacuum tubes. It involves weighting each signal by its respective signal to noise ratio, then adding the signals together. Other sub-optimal approaches have been studied for wireless digital communications, which have lower levels of performance but less implementation complexity compared to the maximal ratio combiner. One approach called “block-based selection” involves simply selecting a block of data with no bit errors, if available from any one of the signal paths. Many digital communication systems include block codes that allow the receiver to detect and/or correct bit errors in blocks of data. Examples are Reed-Solomon (RS) codes and cyclic redundancy check (CRC) codes. Decoding results for these codes can be used in a receiver to perform block-based selection among the outputs of multiple decoders operating on signals from multiple antennas. Although block-based selection cannot match the performance of maximal ratio combining, especially for the case of a flat additive white Gaussian noise (AWGN) channel, it provides significant gain over the single antenna case for many practical scenarios.
Known diversity combiners typically process the signals received from multiple antennas in parallel. However, this approach is limited in that the output of one decoder cannot be used to influence another decoder's operation.
In digital communication systems, it is common to use two levels of error correction coding, an inner code and an outer code, with an interleaver in between. In general, the inner code enables correction of shorter error events, while the combination of interleaver and outer code enables correction of longer error events. For example, in digital television signals transmitted according to standard A/53 from the Advanced Television Systems Committee (ATSC), an inner trellis code is used along with an interleaver and an outer Reed-Solomon code.
In traditional prior art receivers, the inner decoder and outer decoder operate independently, with a de-interleaver in between. However, these traditional receivers typically operate several dB away from the Shannon limit for their respective data rates and signal bandwidths. For example, the best traditional ATSC receivers typically can handle about 14.9 dB carrier-to-noise ratio (C/N) at the Threshold of Visibility (TOV), whereas the Shannon limit is about 10.5 dB. Part of this difference can be attributed to limitations of the trellis and Reed Solomon (RS) codes themselves. However, a significant part of this difference is due to the traditional decoder architecture, which does not fully utilize the power of the trellis and RS codes. An example of such a prior art system is shown in FIG. 1.
FIG. 1 shows a prior art decoder 10 to include an inner decoder 12, receiving its input from a signal processing circuit (not shown), coupled to a de-interleaver 14, which is shown coupled to an outer decoder 16. The inner decoder 12 typically utilizes trellis decoding techniques, for example the Viterbi algorithm, to partially decode the signal received from the signal processing circuit and the outer decoder 16 typically uses RS decoding techniques, for example the Berlekamp-Massey algorithm, to decode the remainder of the coding present in the received signal. The outer decoder 16 provides the signal that is sought to be received and decoded. Note that the ATSC A/53 system includes a randomizer, so the outer decoder output must also be processed by a de-randomizer (not shown) to generate the final output of the receiver. No matter how optimal their decoding algorithms are, the inner and outer decoders 12 and 16 cannot fully utilize the power of the concatenated coding techniques, because the inner and outer decoders 12 and 16 operate independently of each other.
Attempts have been made to harness the full power of concatenated codes such as those used in ATSC A/53 transmission. In some prior art systems, an iterative decoder involves re-interleaving, re-encoding, and re-mapping an outer decoder output to generate known inputs to a subsequent inner decoder and is in reference to Direct Broadcast Satellite (DBS) standards. A limitation in the foregoing prior art system is that the re-encoding process can have infinite memory, so an error in the outer decoder output can cause the re-encoded output to be wrong from that point forward. In some transmission systems, this issue may be mitigated by the fact that the encoder state is reset at regular intervals. However, this is not the case in many transmission systems, including that defined in ATSC A/53.
Other prior art techniques use parity information from an outer decoder to improve performance of a subsequent inner decoder and lay claim to enabling decoding of the ATSC A/53 signal at 14.6 dB C/N, a gain of 0.3 dB. Although some information from the outer decoder is used to improve inner decoder performance, this technique does not fully utilize all of the information. For example, it does not make use of the reliably corrected data bits available at the output of the Reed-Solomon decoder.
In yet other prior art techniques, an iterative decoder involves re-interleaving a “marked decoded output” from an outer decoder, and using it to discount states in a subsequent inner decoder and lay claim to a gain of about 1.0 dB in C/N performance for codes used in Digital Video Broadcasting (DVB) standards. The primary cost associated with this gain is the extra memory required to store delayed inputs and perform re-interleaving. Each decoder iteration, in the foregoing technique, requires a large amount of extra memory, and no mechanism is provided for trading off memory size and performance for a given number of iterations.
Moreover, all of the prior art iterative decoders described above are directed to single input scenarios.
In light of the foregoing, there is a need for a diversity combiner that makes use of iterative decoding that more fully utilizes the power of its concatenated codes, and that reduces memory size while maintaining or improving performance. There is further a need for such a diversity combiner to be applicable to signals transmitted according to ATSC A/53.